A. In my last Q&A, I began this month's exploration of VMware HA's admission control. I also discussed why it's a necessary performance management component of HA. Admission control's primary job is to maintain a quantity of unused resources (such as processing and memory) in reserve. That reserve ensures that any host can fail and its virtual machines (VMs) will power on elsewhere without affecting the performance of others.
That said, there are some situations where turning on admission control might not be the best idea. You might want the failover capability VMware HA provides but not want to hold that amount of processing and memory resources unused and in reserve.
One situation where this can be the case is when your ESX host cluster isn't made up of a large number of hosts. A cluster of only a few ESX hosts, as you can imagine, requires a greater percentage of its total resources to be set aside.
Think about the concept of cluster reserve. When a cluster host fails, its VMs must fail over to other hosts in the cluster. Typically, situation you'd want enough processing and memory resources to be available and unused elsewhere in the cluster to support failover. At the same time, because of VMware's memory and processor oversubscription features, it's possible for VMs to fail over even if there aren't enough resources for all of their needs. The obvious result, should this oversubscription, happen is that all VMs will suffer a performance penalty.
For the sake of argument, I'll call these set-aside resources "wasted resources." In a small ESX cluster of only four hosts, admission control might set aside 25 percent of the cluster's total resources. That 25 percent represents one host's resource contribution. It also represents a lot of wasted resources in comparison with your total hardware investment. On the other hand, a cluster of ten hosts might only need 10 percent of its total resources—a much lower overall percentage of waste. As such, it's probably a situation your budget will tolerate.
It's here that disabling admission control becomes an acceptable option. If your budget is tight and your cluster hosts are small in number, you might not want that level of wasted resources. You might be OK with the idea that a host failure and subsequent VM failover will cause a performance impact to all VMs. If this is the case, you might consider enabling VMware HA but disabling admission control.
Coming Up! Don't like the idea of disabling admission control? I'll show you an alternate configuration that might work better!
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